1. Field of the Invention
This invention relates generally to servo systems for positioning transducers, such as magnetic heads, in disk drives and more particularly to such servo systems in which provision is made for correcting variations in transducer outputs for eliminating variations in servo gain.
2. Description of the Prior Art
In disk drives, variations in recorded servo code, variations in the dimensions of magnetic poles on magnetic heads, and variations in electromagnetic properties among the magnetic heads cause variations in the induced signals which result in variations in servo gain. This adversely affects servo performance in track seeking and track following functions.
In these disk drives, the magnetic heads are mounted on flexure assemblies. Each flexure assembly comprises a load beam and a thin flexure on the end of the load beam which mounts the magnetic head and which functions as a gimbal permitting pitch and roll of the magnetic head while providing directional stiffness therefor. Each surface of each disk of the disk drive is scanned by a magnetic head. Thus, depending upon the number of disks, two or more such flexure assemblies comprise part of an arm stack of an actuator, either of the linear or rotary type, for moving the magnetic heads to different tracks for reading or writing purposes.
The arm stack is powered by a motor, for example, a voice coil type of motor which is part of a servo. The servo is controlled by a microprocessor which converts requests for information from a host computer into disk, head, track and track sector selections, providing voice coil power and response to head feed back in track seeking, track following and read/write functions.
Disk drives are precision mechanical devices, providing track seeking and track following functions in track densities of typically 2,000 tracks per inch and higher. In such a mechanical environment, position transducer signal precision, particularly in regard to signal gain, is also required, to minimize servo gain variations.
Although the fabrication of the magnetic heads and the recording of servo code on the disks are accomplished with high precision. There is sufficient variation in the physical dimensions and the electromagnetic properties among individual heads to result in undesirable signal variations among the heads in demodulating the servo position signal. Likewise, the same magnetic head in scanning different tracks of servo code may show undesirable-signal gain variations.
Examples of different approaches presently known to the applicants, which have been taken in the past in achieving compensation for variations in servo code are described in U.S. Pat. Nos. 4,688,118, 4,786,990, and 4,823,212, which are discussed below. All of these patents are assigned to the assignee of this invention.
U.S. Pat. No. 4,688,118 provides compensation for signal gain by employing individual magnetic heads to record special servo code patterns and thereafter to read the servo code patterns which it has recorded. Thus, magnetic heads which may have different widths and different electromagnetic properties "see" the same amount of servo code and experience the same electromagnetic coupling to minimize servo gain.
U.S. Pat. No. 4,786,990 achieves servo gain variation compensation among the heads of a disk drive by detecting individual servo gain corrections which are required for each magnetic head at several different tracks on each disk surface. These corrections are stored and are accessed each time a particular head is selected to provide servo gain variation compensation at a particular track location for that magnetic head.
U.S. Pat. No. 4,823,212 provides an AGC field ahead of the embedded servo code in each track on each disk surface, so that during track seeking and in advance of track following the servo gain reading for that head can be determined thereafter, the track following function for that head takes place.
All of the patents aforesaid describe approaches to compensating for servo gain variations at the source, namely the magnetic head, and thus represent useful solutions to the problem of servo gain variation.
Further improvement however, is realized along with simplification in the implementation of servo gain variation compensation in the approach provided by the present invention.